1. Field of the Invention
The present invention relates to twist drills and, more specifically, to a twist drill with a thick web not requiring thinning.
2. Description of the Prior Art
Recently, it has been strongly desired to enhance the efficiency of the twist drilling process and to improve drilling accuracy. Generally, twist drills having enhanced rigidity are employed for improving drilling efficiency.
As illustrated in FIG. 1, a twist drill 1 having an enhanced rigidity has a web of increased thickness, as compared with the web of a standard twist drill indicated by broken lines, formed by expanding the margin lips 3 and the heels 4 to enhance rigidity. Such a twist drill having a thick web, however, has a chisel edge longer than that of the standard twist drill, and hence the center axis of rotation of the twist drill is liable to deviate from a correct position so as to deteriorate the drilling accuracy due to walking, the cutting performance is deteriorated due to an increased cutting resistance, particularly, the thrust. Accordingly, it is usual practice to subject such a twist drill having a thick web to a thinning process to shorten or to remove the chisel edge as illustrated in FIG. 2. Twist drills based on such conception are disclosed, for example, in Japanese patent publication Nos. 58-18163 and 61-30845. These known twist drills require high thinning skill because the thinning accuracy affects drilling accuracy directly. These known twist drills have a further problem in that they need to be subjected to a thinning process after each regrinding.
Furthermore, since the volume of the flutes of the known twist drills having a large web thickness is smaller than that of the standard twist drills, the chip removing performance thereof is deteriorated.